The present invention relates to an illumination unit and, more particularly, to an astral lamp used in dental and other medical treatments.
An illumination unit used in dental and other medical treatments is designed to avoid generation of a shadow in the illumination area, and is accordingly usually called an astral lamp. As shown in FIG. 26, an astral lamp of this type has a heat-resistant-glass reflecting mirror, in this case, a parabolic mirror 52 of revolution, an arm 59, a protection cover 57, a light source light-shielding cylinder 58, and a light source 51. The parabolic mirror 52 has a plurality of segments 53 made of flat mirrors. The arm 59 supports the parabolic mirror 52. The light source 51 is comprised of a linear halogen lamp or other linear light source, and will be referred to as a linear light source hereinafter. Generally, the linear light source 51 is arranged in front of the focal point of the parabolic mirror 52.
The parabolic mirror 52 of revolution is formed into a concave mirror along a concave paraboloid of revolution formed by rotating a predetermined parabola about its vertex as the center. The parabolic mirror 52 reflects light emitted by the linear light source 51 toward the linear light source 51 to form a light path 55. The light path 55 condenses light toward a predetermined illumination area 56 remote from the linear light source 51, thereby irradiating only the specific portion, i.e., morbid portion of a patient. Even if the linear light source 51, the doctor""s hand, or other light-shielding object enters the light path 55 to partially block light, the light path 55 must be able to ensure a high shadowless degree and illumination uniformity. A xe2x80x9cshadowless degreexe2x80x9d is a degree with which, even if a light-shielding object enters a light path having a predetermined illumination area, a shadow image is not formed in the illumination area. An xe2x80x9cillumination uniformityxe2x80x9d is a degree with which the reflected light beam is diffused uniformly and theoretically within the illumination area.
The size of the illumination area 56 of the light path 55 is determined by the position of the linear light source 51. More specifically, when the linear light source 51 is arranged at the focal position of the parabolic mirror 52, the light beam reflected by the parabolic mirror 52 forms parallel light substantially parallel to the axis of rotation of the paraboloid of revolution, i.e., the optical axis of the parabolic mirror 52, so that the size of the illumination area 56 becomes substantially equal to or larger than the opening area of the parabolic mirror 52. When the linear light source 51 is arranged behind the focal position, the reflected light beam is diffused, and the size of the illumination area 56 becomes larger than the opening area of the parabolic mirror 52. Inversely, when the linear light source 51 is arranged in front of the focal position, the reflected light beam is condensed, and the size of the illumination area 56 becomes smaller than the opening area of the parabolic mirror 52.
Usually, when an astral lamp is used for dental treatment, the linear light source 51 is arranged in front of the focal point to reduce the light path 55 toward a desired illumination area 56 smaller than the parabolic mirror 52.
As the reflecting mirror of such an astral lamp, various types are conventionally proposed, and among them, prior arts disclosed in Japanese Utility Model Publication Nos. 61-25123 and 60-31695, Japanese Utility Model Laid-Open No. 3-88215, and the like are known.
In a reflecting mirror for an astral lamp described in Japanese Utility Model Publication No. 61-25123 (to be referred to as prior art 1 hereinafter), as shown in FIG. 27, in order to obtain a high shadowless degree and illumination uniformity, a large number of segments 53 formed of rectangular flat mirrors are formed on the fundamental paraboloid of a parabolic mirror 52 of revolution divisionally in the direction of the major axis of the parabolic mirror 52 of revolution. Each rectangular flat mirror segment 53 has a long side coinciding with the minor axis of the fundamental paraboloid. The short width of each segment is set to a value corresponding to the major-axis width of the light path formed by the astral lamp.
In a reflecting mirror for an astral lamp described in Japanese Utility Model Publication No. 60-31695 (to be referred to as prior art 2 hereinafter), in order to similarly obtain a high shadowless degree and illumination uniformity, a large number of segments formed of flat mirrors are formed on the fundamental paraboloid of a parabolic mirror of revolution divisionally in the main direction (major-axis direction) and the subdirection (minor-axis direction) of the parabolic mirror of revolution. The long and short widths of each segment are set to values respectively corresponding to the major- and minor-axis widths of the light path. In other words, each segment is formed into such a size that it can diffuse the reflected light beam to reach the illumination area of the light path.
In an astral lamp described in Japanese Utility Model Laid-Open No. 3-88215 (to be referred to as prior art 3 hereinafter), in order to diverge the light in the subdirection and main direction, a large number of convex reflecting surfaces are aligned on the inner surface of a parabolic mirror of revolution or elliptic mirror of revolution in the main direction and subdirection. The vertical width (short width) of each convex reflecting surface is set smaller than the horizontal width (long width) thereof.
As another conventional unit, an astral lamp for dental treatment disclosed in Japanese Patent Laid-Open No. 2-65856 (to be referred to as prior art 4 hereinafter) is known. According to this prior art 4, the reflecting mirror is constituted by a curved surface portion corresponding to an operation field and having a function of condensing light to increase the luminous intensity, and a curved surface portion corresponding to a peripheral part of the operation field and having a function of dispersing light to lower the luminous intensity. The curved surface portion having the condensing function is formed of a spherical surface, a paraboloid, an ellipsoid of revolution, or the like. The curved surface portion having the light dispersing function is formed of a spherical surface having a radius larger than that of the curved surface portion having the condensing function.
Any one of the conventional prior arts 1 to 4 is still insufficient to obtain a high shadowless degree and illumination uniformity.
In prior art 1, as shown in FIG. 28, a fundamental paraboloid 54 that forms the inner surface of the parabolic mirror 52 is a paraboloid of revolution having a constant focal point, e.g., a focal length Fa. A plurality of segments 53 formed of flat mirrors are formed on the fundamental paraboloid 54 divisionally in the main direction of the fundamental paraboloid 54. Each segment 53 forms a paraboloid identical to the fundamental paraboloid 54 in the direction of the long side.
Light reflected by any point of the parabolic mirror 52 must form an illumination area, even at a position far from the parabolic mirror 52, to have a desired width smaller than the sub-direction width of the parabolic mirror 52, and condense the reflected light toward the illumination area at high precision. However, since each segment 53 is formed along one fundamental paraboloid 54 in its long-side direction, these two requirements cannot be satisfied.
In FIG. 28, the segment 53 is formed by using the fundamental paraboloid 54 that enables light reflected by any point of the parabolic mirror 52 to have a desired width in the subdirection within the illumination area far from the parabolic mirror 52. When, however, compared to an illumination area 56c formed by reflected light 55c reflected at an arbitrary point P3 on the segment 53 which is far from a linear light source 51, an illumination area 56a formed by light 55a reflected at an arbitrary point P1 near the linear light source 51 is undesirably shifted downward. Although FIG. 28 shows only the upper half of the parabolic mirror 52, in the lower half, the reflected light is shifted upward, in the opposite manner to that described above. Accordingly, the luminous intensity of the illumination area is highest at the central portion and decreases upward and downward. A high shadowless degree cannot be obtained, and the illumination area cannot be irradiated at a high illumination uniformity.
Prior art 2 is different from prior art 1 in that the plurality of segments formed of flat mirrors are formed on the fundamental paraboloid of the parabolic mirror of revolution divisionally in the main direction and subdirection. In this structure as well, the fundamental paraboloid of the parabolic mirror of revolution is formed by using one paraboloid having a predetermined focal length. If each reflected light beam is to have a desired width in the subdirection within the illumination area in the same manner as in prior art 1, the reflected light is undesirably shifted in the subdirection. As a result, the illumination area cannot be irradiated at a high illumination uniformity.
In prior art 3, light diverges in the horizontal and vertical directions by a large number of convex reflecting surfaces, so that a large illumination area is obtained. Accordingly, the luminous intensity of the illumination area decreases.
Prior art 4 is different from prior arts 1 and 2 described above in that the curved surface portion having the function of condensing light to increase the luminous intensity of the operation area is formed of merely a spherical surface, a paraboloid, an ellipsoid of revolution, or the like, and is not divided. However, since the radius of curvature or focal length of the curved surface portion is constant, the reflected light is undesirably shifted, in the same manner as in prior arts 1 and 2, and the illumination area cannot be irradiated at a high illumination uniformity.
The present invention has been made to solve the conventional problems described above, and has as its object to provide an astral lamp which can condense light reflected at different points toward a desired illumination area efficiently, so that a high shadowless degree and illumination uniformity can be obtained.
In order to achieve the above object, according to the present invention, there is provided an astral lamp comprising a light source and a concave mirror for reflecting light emitted by the light source and condensing the reflected light on a light source side toward an illumination area remote from the light source, the concave mirror being constituted by a plurality of concave mirror surfaces that form one parabolic mirror of revolution as a whole, and the mirror surfaces respectively having curved surfaces for separately reflecting the light emitted by the light source and condensing the reflected light toward an entire portion of the illumination area.